1. Field of the Invention
The present invention relates to a video signal reproduction apparatus for reproducing with de-emphasis a video signal subjected to pre-emphasis characteristics of which a level of high frequency components of the video signal is increased relative to low frequency components thereof in the time axis direction (time domain).
2. Description of the Prior Art
When a video signal is recorded on a recording medium such as magnetic tape, it has been usual, in order to minimize effect of noise in recording and reproducing operations or to improve an SN ratio, to record and reproduce the video signal by subjecting it to a frame emphasis so-called a temporal emphasis. That is, the video signal is recorded after frame pre-emphasis by which high frequency components of the video signal in time axis direction, are emphasized relative to low frequency components thereof every frame period and is reproduced with frame-de-emphasis which attenuates the high frequency components of a reproduced video signal relative to the low frequency components every frame period.
FIG. 1 shows a block circuit diagram of a video signal recording and reproducing apparatus which employs such frame emphasis, FIG. 2 shows a block circuit diagram of a pre-emphasis circuit, FIG. 3 is a block circuit diagram of a de-emphasis circuit, FIG. 4 shows a video signal recorded on a magnetic tape after frame pre-emphasis is applied and FIG. 5 explains a special reproduction of the video signal which is recorded on the magnetic tape after the frame pre-emphasis is applied.
In the video signal recording and reproducing apparatus shown in FIG. 1, a video signal inputted to an input terminal 1 is supplied to a pre-emphasis circuit 2 and a frame pre-emphasized video signal is supplied to a recording signal processing circuit 3 in which the video signal is suitably processed by such as frequency modulation, etc. The video signal thus processed by the recording signal processing circuit 3 is supplied through a recording amplifier (not shown), etc., to a magnetic head (not shown) by which the signal is recorded on a magnetic tape 4.
In the reproduction thereof, on the other hand, the recorded video signal on the magnetic tape 4 is reproduced by a magnetic head (not shown). After being amplified by a reproduction amplifier (not shown), the reproduced signal is processed in a reproducing signal processing circuit 6 in a reverse manner to the recording signal processing, and is supplied to a de-emphasis circuit 5 in which the reproduced video signal is frame de-emphasized, and subsequently, the frame de-emphasized reproduced signal is derived from an output terminal 7.
The pre-emphasis circuit 2 is constituted with a frame memory 13, coefficient circuits 14 and 15 having coefficients Kp and hp respectively, subtractor circuits 17 and 18 and an adder circuit 19, as shown in FIG. 2.
The de-emphasis circuit 6 is constituted with a frame memory 23, coefficient circuits 24 and 25 having coefficients Kd and Ad respectively, and subtractor circuits 27, 28 and 29, as shown in FIG. 3.
The frame pre-emphasis characteristics of the pre-emphasis circuit 2 becomes inverted with respect to the frame de-emphasis characteristics of the de-emphasis circuit 6, that is, a product of their transfer functions becomes 1, by selecting the coefficients Kp, Ap, Kd and Ad as follow:
Kp=0.422, Ap=2.11 PA1 Kd=0.679, Ad=0.566
In this case, the transfer function between the input terminal and the output terminal of the video signal recording and reproducing apparatus shown in FIG. 1 coincides with the transfer function without frame pre-emphasis and frame de-emphasis since the frame pre-emphasis characteristics and frame de-emphasis characteristics thereof are cancelled out each other.
Therefore, the reproduced video signal is not affected by the frame emphasis and thus the reproduced video signal at the output terminal 7 is the same as the signal without frame emphasis.
Since the above mentioned frame de-emphasis functions in the same manner as that of the well known frame recursive noise reducer, noise appearing during recording on the magnetic tape 4 is suppressed.
The frame emphasis operation will be described on a recording format on the magnetic tape 4.
When a video signal is recorded by means of the video signal recording and reproducing apparatus shown in FIG. 1, it is arranged on the magnetic tape 4 in sequence of frames as shown in FIG. 4.
Since, in the frame pre-emphasis processing, there is a feedback loop which bridges two adjacent frames as shown in FIG. 2, an influence of the frame pre-emphasis will appear between the adjacent frames. For example, a frame Pn in FIG. 4 contains information of the frame Pn-1 and frames preceding the latter.
When this magnetic tape is reproduced, these frames are reproduced in time sequence. Therefore, when the frame de-emphasis is performed on the frame Pn, it is impossible to frame de-emphasize the frame Pn alone since the information of the frame Pn-1 and the preceding frames have been stored in the frame memory 23.
There presents problems when a special mode reproduction is performed on the signal recorded on the magnetic tape with a frame emphasis, if the reproduced signal undergoes a frame de-emphasis for a normal mode reproduction.
Modes of such special reproduction may include a still (picture) reproduction, a frame (picture frame) advance reproduction or a slow motion reproduction. These special reproductions will be described with reference to FIGS. 5(a) to 5(c), which show video signals recorded on the magnetic tape 4 after being subjected to a frame pre-emphasized operational process in the frame order of F1, F2, F3, F4, F5, . . . , respectively.
(1) Still Reproduction
When the frame F1 on the magnetic tape 4 in FIG. 5(a) is still-reproduced by repeatedly reproducing the same frame (or the field), the frame de-emphasis is repeated for the same frame (or the field).
Therefore, in this case, the frame emphasis applied in the recording is not cancelled out (not properly complemented) by the frame de-emphasis in the reproduction, as a result, a reproduced video signal of the frame F1 underwent the pre-emphasis only is outputted, which provides an irregular image on a screen.
(2) Frame Advance and Slow Motion Reproductions
When the frame F1 shown in FIGS. 5(b) and 5(c) is reproduced in the frame-advance or the slow motion, a reproducing period of the frame F1 is extended longer than that in the normal reproduction.
Therefore, during this extended time period, the same frame (or the field) undergoes frame de-emphasis repeatedly.
Therefore, in this case, the frame emphasis applied in the recording is not cancelled out (not properly complemented) by the frame de-emphasis in the reproduction, causing that a reproduced video signal of the frame F1 underwent the pre-emphasis only is outputted, resulting in irregular image.
Further, when the subsequent frame F2 is reproduced in the frame advance or the slow motion mode, the frame de-emphasis operational process is performed in the order of frames F1, F1, F1, . . . , F2 while the frame pre-emphasis in the recording is performed in the order of frame F1, F2, . . . Therefore, operational process becomes no longer corresponding between the pre-emphasis and the de-emphasis resulting in a reproduced image having residual image or blur.